Graphene is a two-dimensional crystal composed of carbon atoms and is a source material that has been attracting considerable attention since it was discovered in 2004. Graphene has excellent electrical, thermal, optical and mechanical properties and is expected to be applied in a wide range of fields such as battery materials, materials for energy storage, electronic devices and composite materials.
In order to realize such applications of graphene, making the preparation method efficient to reduce costs and an improvement in dispersibility are required tasks.
Examples of the production method of graphene include a mechanical exfoliation method, a CVD (Chemical Vapor Deposition) method, a CEG (Crystal Epitaxial Growth) method and the like, and these methods have low productivity and are not suitable for mass production. In contrast, an oxidation-reduction method (a method of subjecting natural graphite to an oxidation treatment to obtain a graphite oxide or a graphene oxide and then preparing graphene by a reductive reaction) is capable of attaining a large-scale synthesis of graphene and is a very important method for putting graphene into practical use.
Examples of the conventional technique of the oxidation-reduction method include a high temperature heat reduction method and a method of reducing with hydrazines. In Patent Document 1, for example, although a graphite oxide is allowed to undergo a heat reduction at a high temperature of 1050° C. to prepare graphene, with regard to the conditions, since the method is conducted at high temperatures, expensive facilities are required. Ruoff, et al. employed hydrazine hydrate as a reducing agent and graphite oxide was reduced by a 24-hour reaction at 100° C. in water to prepare graphene (Non-Patent Document 1). However, hydrazine reducing agents are highly toxic and are difficult to be industrially utilized.
Moreover, since graphene is a nanocarbon and is very difficult to be dispersed due to its high specific surface area, an improvement in dispersibility is a very important task for the application of graphene. Although Li, et al. obtained graphene stably dispersed in water by adding aqueous ammonia during reducing graphite oxide with hydrazine hydrate as a reducing agent to allow graphene to be electrically charged (Non-Patent Document 2), in this method, the solvent is restricted to water.
In Patent Document 2, a relatively stable graphene dispersion was prepared from graphite oxide using phenolamines as reducing agents. Since the graphite oxide cannot be sufficiently reduced with the phenolamines, sufficient conductivity cannot be attained. Although Liu, et al. and Kaminska, et al. employed dopamine (Non-Patent Document 3) and an azide of dopamine (Non-Patent Document 4), respectively, as a reducing agent to reduce graphite oxide, since the graphite oxide could not be sufficiently reduced with the dopamines, graphene with high conductivity could not be attained.